Shimon – that’s exactly why goyim can marry their sisters according to many rishonim – baruch shekivanta. There is a rational reason however not to, which is that people have a disgust of such a marriage…it also results in birth defects.
I think eiver min hachai is very rational; Hashem does not want us to build bad middos in ourselves, and torturing animals or benefitting from their direct unnecessary suffering will do that.
CT – I should have been clearer. In theory theres no limit to the bechira in and of itselr. The limiting factor is Hashem’s control over our actions, versus what he lets us decide to do. The baalei machshava, including the ramchal and chovos halevavos (I’ll have to get the sources if need be) say that we only control that which has a moral significance. For a Jew, this includes virtually everything we do, because the chovos halevavos says that there’s no such thing as a “pareve” action – everything ultimately is either doing what Hashem wants or the opposite. I realize this may be a chiddush to some and i will imyh get the mekoros when i have some free time.
It follows then that goyim make far less decisions which have moral implications, therefore a lot of what they think they’re deciding is just Hashem running the world and using them as shluchim.